Abstract

This thesis investigated a range of interdependent themes. There were two central questions. The first addressed whether visual information for the control of skilled actions is transformed and represented in a similar fashion as that used to represent the world for perception. The second addressed the nature of the visual information used in the control of reaching and grasping movements. A MacReflex (Qualisys AB) motion analysis system was used to measure kinematic parameters of reaches to real-world objects in a variety of situations. The results of the first empirical chapter, which explored the effects of pre-response delay on performance on various perception- and action-based tasks, supported Milner and Goodale's (1995) contention that visual processing for perception and for the control of action are segregated in two quasi-independent processing streams. Three further empirical chapters investigated the importance of binocular cues relative to monocular pictorial cues in the control of prehensile movements. Binocular cues were found to be sufficient but not necessary for the control of reliable reaches, both when reaching to single objects in isolation and when reaching into multiple-object scenes. This finding questions the current view that they play a primary role in the control of reliable reaches. Height in the visual field, which can, in principle, specify metric properties of the scene, was also found to be sufficient to support reliable reaching. The final empirical chapter addressed the implications of restricting peripheral information for estimates of distance and size used in the control of reaches. It was found that restricting the field of view resulted in underestimates of object distance although estimates of size were unchanged. In summary, the findings reported in this thesis support the dissociation between visual processing for perception and action, as proposed by Milner and Goodale (1995) and suggest that a variety of binocular and monocular sources of information are used in the control of natural reaching and grasping movements.